Method for preparing titanium and titanium alloy surface modification layer through magnetic field assisted laser remelting

Abstract

The invention discloses a method for preparing titanium and titanium alloy surface modification layers through magnetic field assisted laser remelting. Comprising the steps that a titanium plate is polished through abrasive paper, cleaned through ethyl alcohol and dried through hot air; fixing the titanium plate and the permanent magnet in a laser processing platform; adjusting the distance between the fixed magnet and the titanium plate, and adjusting the magnetic field intensity and the magnetic induction line direction; and in a specific atmosphere, laser processing equipment is started, the surface of the titanium plate is remelted under the auxiliary action of a magnetic field, and the titanium alloy surface coating is prepared. According to the invention, the energy of the laser spot is redistributed by utilizing the restraint effect of the magnetic field on the light-induced plasma, so that the energy in the spot is more uniform, and the more uniform melting depth is obtained. Due to the fact that thermomagnetic current and the like are generated in the molten pool, the magnetic field restrains flowing, fluctuation and the like of the molten pool, and the quality of the obtained remelting surface is higher. According to the method, the effect of the magnetic field on the light-induced plasma and the molten pool fluid is utilized, the melting zone and the heat affected zone of laser remelting are reduced, the surface fluctuation of the modified layer is inhibited, the surface roughness is reduced, and the preparation quality of the titanium alloy surface modified layer is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser surface modification, in particular, especially to a method for preparing a titanium and titanium alloy surface modification layer by magnetic field assisted laser remelting. BACKGROUND

[0002] Titanium and titanium alloy has low density, high specific strength, no magnetism and non-toxicity, good plasticity and toughness, and good weldability, and is widely used in marine vessels, aerospace, biomedical materials and other fields. However, titanium and titanium alloy has low hardness and poor wear resistance, and is prone to adhesive wear and abrasive wear during abrasive process. Oxygen atoms have high solubility in α-Ti and can produce significant solid solution strengthening effect to improve surface hardness. Rutile TiO2 and other oxides have higher hardness than titanium alloy, so preparing a hard oxide film composed of an oxide layer and an oxygen diffusion layer on the surface of titanium alloy can improve the hardness and wear resistance of titanium alloy. Titanium alloy is widely used in marine equipment due to its excellent mechanical properties, such as ship body structural parts, submarine pressure hulls, propellers, and pipelines. Seawater is an electrolyte with extremely strong corrosive properties in nature. In seawater environment, titanium alloy is prone to contact with materials with high corrosion potential such as steel and copper, resulting in galvanic corrosion and causing serious damage. By preparing an oxide film with high resistivity on the surface of titanium alloy, the conductive contact can be isolated, and the oxidation resistance and corrosion resistance of titanium alloy can be improved. Preparing a titanium oxide coating on the surface of medical titanium alloy can improve the biocompatibility while reducing the diffusion of toxic elements in titanium alloy into the body.

[0003] Titanium and titanium alloy can be rapidly heated and cooled by laser under different atmospheres to prepare surface oxide or nitride layers, promote the diffusion of oxygen and nitrogen elements into the substrate, and improve the surface hardness, corrosion and wear resistance. Compared with thermal oxidation and other processes, laser scanning for preparing a modified layer can reduce the influence of high temperature on the substrate, reduce the deformation of the workpiece, and improve the processing efficiency. However, due to the Gaussian distribution of laser energy in the spot, the depth of the molten pool center obtained by laser remelting is greater than that of the edge, and the heat-affected zone corresponding to the center of the spot is greater than that of the edge, increasing the melting and heat-affected range of the substrate. Moreover, after remelting and solidification of titanium and titanium alloy, ripples and undulations are generated on the surface due to surface tension and other reasons, resulting in increased roughness. Therefore, it is necessary to improve the laser remelting process to further reduce the heat-affected zone during the preparation of titanium alloy coating and improve the surface quality. SUMMARY

[0004] In view of the above problems, the application provides a method for preparing a surface modification layer of titanium and titanium alloy by magnetic field assisted laser remelting. The method promotes the aggregation of photo-induced plasma above the titanium alloy by applying a steady magnetic field, reduces the energy absorption of the substrate to the center of the light spot, and realizes the homogenization of the depth of the molten pool. Due to the generation of thermoelectric current and the like, the steady magnetic field can inhibit the flow of the molten pool metal, reduce the ripples and undulations on the surface of the molten titanium alloy, and improve the roughness of the remelted surface.

[0005] The technical scheme of the application is as follows: a method for preparing a surface modification layer of titanium and titanium alloy by magnetic field assisted laser remelting, comprising the following steps:

[0006] S1: sandpaper polishing, alcohol cleaning and hot air drying are performed on the workpiece to be machined.

[0007] S2: the workpiece is placed in a specific steady magnetic field generated by a permanent magnet, is fixed, and is placed in a laser processing platform.

[0008] S3: a continuous laser is used, the power is 600-2000W, the power output stability is less than 2%, the laser is positively defocused by 0-15mm, the spot diameter is 3-6mm, the scanning speed is 10-50mm / s, the laser incidence angle is perpendicular to the substrate surface, and the off-axis gas flow is 15-20L / min.

[0009] S4: the laser is turned on in a specific atmosphere, and the remelting of the substrate surface is completed under the synergistic action of the magnetic field to prepare a surface modification coating.

[0010] Further, the workpiece in step S1 is titanium or titanium alloy.

[0011] Further, the steady magnetic field in step S2 is established by a permanent magnet, and the magnetic field direction includes two kinds: perpendicular to the surface of the workpiece and horizontal and perpendicular to the scanning path.

[0012] Further, the permanent magnet in step S2 is large enough, or a plurality of permanent magnets are used to establish a magnetic field direction consistent with the direction of the surface to be machined.

[0013] Further, in step S2, the distance between the permanent magnet and the workpiece is adjusted to change the magnetic field strength of the surface to be machined of the workpiece. The distance between the permanent magnet and the workpiece is 0-10mm, and the magnetic field strength of the surface to be machined is 0.01-0.3T.

[0014] Further, in step S3, the off-axis sweeping gas is air or atmosphere.

[0015] Further, in step S4, the workpiece is naturally cooled after laser processing.

[0016] Compared with the prior art, the application has the following beneficial effects:

[0017] The application changes the energy absorption of the substrate in the laser spot by restraining the photo-induced plasma on the substrate surface by the steady magnetic field, so that the remelted area is more uniform in the thickness direction, the melting area and the heat affected zone are reduced, and the influence of laser remelting on the substrate is reduced. By restraining the fluid flow of the molten pool by the steady magnetic field, the surface corrugation and undulation after remelting of titanium and titanium alloy surface are inhibited, and the surface quality after remelting is improved. Under the auxiliary action of the magnetic field, a flat and uniform coating is prepared on the surface of titanium and titanium alloy by laser remelting.

[0018] The method used in the application can change the gas application according to the requirements of generating the coating, and the preparation process is fast, low in cost and green and environmental protection. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a schematic diagram of the processing method of the magnetic field assisted laser remelting for preparing a modified layer of titanium and titanium alloy according to the embodiment of the application: (a) schematic diagram of the experimental device, (b) schematic diagram of the transverse magnetic field device, and (c) schematic diagram of the longitudinal magnetic field device. Figure 2 is an ultra-depth-of-field surface morphology diagram of the titanium TA2 surface after remelting modification: (a) surface morphology of the laser remelted modified layer prepared without magnetic field, and (b) surface morphology of the laser remelted modified layer prepared with magnetic field. Figure 3 is a micro-morphology and surface XRD test pattern of the remelted titanium TA2 surface: (a) micro-morphology of the remelted titanium TA2 surface, and (b) XRD test pattern of the remelted titanium TA2 surface. Figure 4 is a micro-morphology of the laser remelted titanium TA2 molten pool: (a) molten pool morphology obtained by laser remelting without magnetic field, and (b) molten pool morphology obtained by magnetic field assisted laser remelting under the same laser parameters.

Claims

1. A method for preparing a surface modification layer of titanium and titanium alloy by magnetic field assisted laser remelting, characterized in that, The method is performed according to the following steps: S1 polishing the workpiece to be processed with sandpaper, alcohol cleaning, hot air drying; S2 placing the workpiece in a stable magnetic field generated by a permanent magnet, fixing and placing in a laser processing platform; S3 turning on the laser, and completing the remelting of the titanium plate surface in a specific atmosphere under the synergistic effect of the magnetic field to prepare a surface modification layer.

2. The method for preparing a surface modification layer of titanium and titanium alloy by magnetic field assisted laser remelting according to claim 1, characterized in that, The workpiece material in step S1 is titanium or titanium alloy.

3. The method for preparing a surface modification layer of titanium and titanium alloy by magnetic field assisted laser remelting according to claim 1, characterized in that, The magnetic field on the surface of the workpiece in step S2 is a stable magnetic field with consistent direction, which is realized by a large enough permanent magnet or a combination of multiple permanent magnets.

4. The method for preparing a surface modified layer of titanium and titanium alloy by magnetic field assisted laser remelting according to claim 1, characterized in that, The magnetic field in step S2 includes a vertical magnetic field perpendicular to the titanium plate and a horizontal magnetic field perpendicular to the laser scanning path.

5. The method for preparing surface modification layers of titanium and titanium alloys by magnetic field-assisted laser remelting according to claim 2, characterized in that, The vertical magnetic field intensity of the titanium plate surface is controlled by adjusting the distance between the titanium plate and the permanent magnet, and the adjustment range of the vertical magnetic field intensity is 0.01T-0.30T. The horizontal magnetic field intensity is controlled by adjusting the distance between the two sides of the permanent magnet and the titanium plate, and the adjustment range of the horizontal magnetic field intensity is 0.01T-0.30T.

6. The method for preparing surface modification layers of titanium and titanium alloys by magnetic field-assisted laser remelting according to claim 1, characterized in that, The laser processing power in step S3 is 600-2000W, the positive defocus is 0-15mm, the spot size is 3-6mm, and the scanning speed is 10-50mm / s.

7. The method for preparing surface modification layers of titanium and titanium alloys by magnetic field-assisted laser remelting according to claim 1, characterized in that, The laser in step S3 is a fiber laser, and the laser wavelength is 1080nm.

8. The method for preparing surface modification layers of titanium and titanium alloys by magnetic field-assisted laser remelting according to claim 1, characterized in that, The specific atmosphere in step S3 is realized by blowing with a laser head through a side-shaft gas path, and the blowing gas is air or oxygen.

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